Method for recycling residual solutions comprising phosphorus and device for such a method

ABSTRACT

A method and device for the purification of phosphoric acid contained in a residual solution is provided that includes undesirable volatilizable materials. The residual solution is mixed with a solution of enriched phosphoric acid and is sprayed into a flame of a combustion chamber in order to form a combustion solution P3 of phosphoric acid purified of the undesirable volatilizable materials. The combustion gases resulting from the combustion chamber are contacted with a feed solution in a gas-acid contactor in order to increase the temperature and concentration of P 2 O 5  and thus to form the solution of enriched phosphoric acid. A portion of this solution is conveyed at a flow rate Qp into the combustion chamber. The remainder of the solution is conveyed into a recycle loop in order to be reintroduced into the gas/acid contactor at a flow rate Q2. The ratio of the flow rates, Qp/(Qp+Q2) is controlled at a predefined value.

FIELD OF THE INVENTION

The present invention relates to a process and a device for purifying residual phosphoric acid solutions derived, for example, from industrial or agro-industrial processes and comprising phosphorus in the form of species such as orthophosphate and/or polyphosphate, and also to undesirable volatilizable materials which make their subsequent use difficult, or even impossible. The process of the present invention is particularly efficient and makes it possible to purify residual phosphoric acid solutions of their undesirable volatilizable materials and, where appropriate, to produce phosphoric acid solutions in high concentrations while at the same time limiting the energy consumption required for the production of purified phosphoric acid or polyphosphoric acid solutions. The present invention allows better management of the combustion gases produced by the process before they are discharged into the atmosphere.

TECHNOLOGICAL BACKGROUND

Many industrial or agro-industrial processes generate residual aqueous solutions comprising phosphorus. These solutions are termed “residual” since they cannot be used without pretreatment in these same industries. The phosphorus of these solutions may be present in the form of species such as orthophosphate or polyphosphate according to the P₂O₅ content of these solutions. On account of the presence in these solutions of dissolved undesirable volatilizable materials such as fluorine, sulfur, carbon, etc., these residual solutions generally cannot be recycled in processes for producing materials with increased added value, such as solutions of purified phosphoric acid or polyphosphoric acid (=PPA), or in the production of phosphate salts. For this reason, these volatilizable materials are termed hereinbelow as “undesirable volatilizable materials”. At the present time, these high-potential residual solutions are treated as simple waste with very low value and high polluting power.

Polyphosphoric acid (=PPA) is a viscous liquid which may notably be produced from phosphoric acid. PPA has the general formula HO[P(OH)(O)O]]_(n)H, with n>1. When n=2, PPA is commonly known as pyrophosphoric acid; when n=3, it is known as tripolyphosphoric acid. For n>3, it is known simply as polyphosphoric acid, independently of the value of n. PPA may be produced by dehydration and polycondensation of orthophosphoric acid, H₃PO₄, according to equation (1). An aqueous polyphosphoric acid solution is thus obtained in which the distribution of the molecular species depends, inter alia, on the polycondensation temperature, Tpc.

Polyphosphoric acid is usually in the form of linear chains. Cyclic forms such as metaphosphoric acid or branched forms may, however, also exist. As illustrated in FIG. 1, the polycondensation temperature determines the concentration of equivalent phosphoric acid as P₂O₅ units at equilibrium in the liquid state (FIG. 1(a)) and the latter acid determines the distribution of the molecular species (i.e. the distribution of phosphoric acid in various forms of different value n (FIG. 1(c)). Thus, an aqueous phosphoric acid solution with an equivalent concentration of P₂O₅ units of less than about 61% will very predominantly consist of H₃PO₄ molecules. When the equivalent concentration of P₂O₅ units increases, this indicates that the solution comprises more and more polymerized molecules, the value of n increasing with the concentration of P₂O₅ equivalents as indicated in FIG. 1(c)).

The dehydration and polycondensation of a solution of phosphoric acid to polyphosphoric acid requires evaporation of water molecules, which requires heat energy input. Patent EP2411325 B1 reviews a certain number of known processes for producing polyphosphoric acid and describes a novel wet-route process relative to the reviewed processes making it possible to benefit from great energy efficiency and to drastically limit the environmental impact. Said patent describes a device which withstands the very harsh operating conditions for producing polyphosphoric acid, making it possible to limit the maintenance costs and to establish equipment durability and finally to ensure the production of a high-quality polyphosphoric acid without contamination during the manufacturing process.

It would be advantageous to produce a material with high added value such as purified phosphoric acid or polyphosphoric acid from low-value residual solutions. However, the use of such residual solutions in a process for producing purified phosphoric acid or PPA as described in EP2411325 B1 is impossible on account of the presence of undesirable volatilizable compounds in such solutions.

JP2000178014 describes a process for recovering phosphoric acid from recovery solutions, in which a recovery solution comprising phosphorus molecules is incinerated at a temperature of 900 to 1000° C. The combustion gases containing phosphorus molecules are cooled in a cooler and the phosphorus molecules are recovered in the form of phosphoric acid.

The present invention proposes a process that is more efficient than the one described in JP2000178014 for the production of purified phosphoric acid or PPA from residual solutions comprising phosphorus derived generally from industrial processes and which, to date, have simply been treated as waste. The present invention and the advantages thereof are described in greater detail in the following sections.

SUMMARY OF THE INVENTION

The present invention is described in the attached independent claims. Preferred variants are defined in the dependent claims. In particular, the present invention relates to a process for purifying an aqueous residual solution comprising phosphorus molecules and undesirable volatilizable materials, comprising the following steps:

-   -   (a) introducing into a gas-acid contactor a feed stream F0 of a         feed solution P0 which is aqueous and comprising phosphorus         molecules preferably in the form of species such as         orthophosphate at a mass concentration, xp0, of between 0 and         54% equivalent of P₂O₅ units,     -   (b) introducing into the gas-acid contactor a recirculation         stream F2 of recirculated enriched phosphoric acid solution P2,     -   (c) introducing into the gas-acid contactor combustion gases G1,     -   (d) contacting the feed stream F0 and the recirculation stream         F2 and the combustion gases G1 to form in the gas-acid         contactor, on the one hand,         -   an enriched phosphoric acid solution P1 comprising a mass             concentration, xp1, with a P₂O₅ content which is greater             than xp0 (xp1>xp0) and, on the other hand,         -   contacted combustion gases G3,     -   (e) separating the contacted combustion gases G3 from the         enriched phosphoric acid solution P1, and then         -   evacuating the contacted combustion gases G3 from the             gas-acid contactor, and         -   removing the enriched phosphoric acid solution P1 from the             gas-acid contactor (1),     -   (f) forming from said enriched phosphoric acid solution P1, on         the one hand,         -   a recirculation stream F2 of recirculated enriched             phosphoric acid solution P2 to introduce it into the             gas-acid contactor (1) as defined in step (b) and, on the             other hand,         -   a spraying stream Fp of the enriched phosphoric acid             solution P1 to introduce it into a combustion chamber,     -   (g) spraying through a flame burning in the upper part of the         combustion chamber a mixing stream Fm of a mixing solution Pm         comprising phosphorus at a mass concentration, xpm, and         undesirable volatilizable materials, the mixing stream being         formed by, on the one hand,         -   the enriched phosphoric acid solution P1 and optionally, on             the other hand,         -   a residual steam Fr of an aqueous residual solution Pr             comprising a mass concentration, xpr, of at least 1% P₂O₅,             to:         -   evaporate the water and thus concentrate the mixing solution             Pm,         -   optionally oxidize and in any case evaporate the undesirable             volatilizable impurities,         -   form combustion gases G1, and         -   forming a combustion solution P3 having             -   a mass concentration, xp3, of P₂O₅ which is higher than                 the concentration of the mixing solution Pm, and             -   a content of volatilizable impurities which is lower                 than that of the mixing solution Pm,     -   (h) separating the combustion solution P3 from the combustion         gases G1 and         -   recovering the combustion solution P3, and         -   transferring the combustion gases G1 into the gas-acid             contactor (1) as defined in step (c).

The feed solution F0 may comprise a concentration xp0 of between 0.1% and 50%, preferably from 1% to 35%, preferably from 5% to 20% P₂O₅. In certain cases, the feed solution may also comprise undesirable volatilizable materials, but this is not essential, notably in the case where a residual solution Pr comprising undesirable volatilizable materials is added to the combustion chamber, as explained below. The flow rate, Q0, of the feed solution F0 in the contactor expressed in nominal power units [MW⁻¹] of the combustion chamber is preferably between 100 and 3000 kg/(h MW), preferably between 500 and 2500 kg/(h MW).

The enriched phosphoric acid solution P1 is identical to the recirculated enriched phosphoric acid solution P2 and comprises a concentration xp1 of phosphorus which is preferably greater than or equal to 1%, preferably less than 60%, more preferably between 5% and 50%, preferentially between 10% and 40% P₂O₅. The total flow rate, Q1=(Qp+O2), of the solution P1 outside the contactor expressed in nominal power units [MW⁻¹] of the combustion chamber is preferably between 600 and 123000 kg/(h MW), preferably between 1000 and 50 000 kg/(h MW). The ratio, Qp/(Qp+Q2), between the mass flow rate Qp of the spraying stream Fp and the total mass flow rate (Qp+Q2) is preferably less than 50%, preferably less than 10%, preferably less than 5%, more preferably less than 2.5% and in which the ratio Qp/(Qp+Q2) is greater than 0.1%, preferably greater than 0.5%.

The residual solution Pr may comprise a phosphorus concentration xpr of greater than or equal to 2%, preferably at least 5%, more preferably at least 10%, preferably at least 20% P₂O₅. The residual solution Pr comprises a concentration xpv of undesirable volatilizable materials of at least 5 ppm, preferably at least 10 ppm, preferably at least 100 ppm, preferably at least 1%, preferably at least 5%, preferably at least 10%, more preferably at least 25% by weight relative to the total weight of the solution. The flow rate Qr of the residual solution Pr in the combustion chamber expressed in nominal power units [MW⁻¹] of the combustion chamber is preferably non-zero and preferably between 5 and 1500 kg/(h MW), preferably between 400 and 1000 kg/(h MW). If the flow rate Qr of the residual solution Pr in the combustion chamber is zero, then the feed solution P0 must comprise a non-zero concentration xpv of undesirable volatilizable materials, for example of at least 5 ppm, preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, preferably of at least 5%, preferably of at least 10%, more preferably of at least 25% by weight relative to the total weight of the solution.

The ratio Qr/(Qr+Q0) may be between 0 and 99%, preferably between 5% and 90%, more preferably between 10% and 80%, or else between 15% and 45%. The mixing stream Fm may comprise a phosphorus concentration xpm preferably greater than 1% P₂O₅ (xpm>1% P₂O₅), and comprises undesirable volatilizable materials, originating from the residual solution Pr and/or from the feed solution P0. In the present document, the flow rates Q0, Qp and Qr are mass flow rates of the feed P0, enriched phosphoric acid P1 and residues Pr solutions, respectively.

The mixing solution Pm may comprise a concentration xpm of greater than 2%, preferably greater than 5%, more preferably greater than 20%, more preferably greater than 30%, preferably greater than 40%, and more preferably between 45% and 60% P₂O₅. The mixing solution Pm may comprise a concentration xpv of undesirable volatilizable materials preferably of at least 5 ppm, preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, preferably of at least 5%, preferably of at least 10%, more preferably of at least 25% by weight relative to the total weight of the solution. The flow rate Qm of the mixing solution Pm in the combustion chamber expressed as nominal power units [MW⁻¹] of the combustion chamber is preferably between 305 and 3000 kg/(h MW), preferably between 200 and 2000 kg/(h MW).

The combustion solution P3 may comprise a phosphorus concentration xp3 of greater than 1% equivalent of P₂O₅ units, preferably greater than 10%, preferably greater than 25%, particularly preferably greater than 40%, or is preferably between 30% and 76%. The flow rate Q3 of the combustion solution P3 outside the combustion chamber expressed in nominal power units [MW⁻¹] of the combustion chamber is preferably between 240 and 1500 kg/(h MW), preferably between 600 and 3000 kg/(h MW).

The feed stream F0 and recirculation stream F2 may be either mixed before they are introduced into the gas-acid contactor to form a stream of a mixture of the feed solution P0 and of the recirculated enriched phosphoric acid solution P2, or contacted after having been introduced separately into the gas-acid contactor to form a stream of a mixture of the feed solution F0 and of the recirculated enriched phosphoric acid solution P2.

It is preferable for the residual flow rate Qr of the residual solution Pr to be non-zero. The residual stream Fr and the spraying stream Fp can then be either mixed to form the mixing stream Fm before being sprayed in the flame in the combustion chamber, or sprayed separately in the combustion chamber to form the mixing stream Fm in the flame or just before reaching the flame. The residual solution Pr and, optionally, the feed solution P0 and thus the spraying solution Pp comprise undesirable volatilizable materials. It is also possible that only the spraying solution Pp comprises undesirable volatilizable materials, for example in the case of a flow rate Qr=0. It is preferred, however, for the residual flow rate Qr to be non-zero.

The contact between the feed stream F0 and the recirculation stream F2 and the combustion gases G1 in step (d) may take place co-currentwise or counter-currentwise, preferably co-currentwise by flowing from an upper part to a lower part of the gas-acid contactor. During the contact step (d), the ratio (Qg1/(Q0+Q2)) between a mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor and a total mass flow rate (Q0+Q2) of the contact feed stream F0 and the recirculation feed stream F2 introduced into the gas-acid contactor, is preferably between 0.1% and 50%, preferably between 0.5% and 10%, more preferably between 1% and 7%.

The present invention also relates to a device for producing purified phosphoric acid (P3) following a process according to any one of the preceding claims, comprising:

-   -   (A) a combustion chamber having:         -   a spraying inlet in the combustion chamber for introducing             at a flow rate an enriched phosphoric acid solution P1 in             sprayed form into a combustion unit,         -   a residue inlet in the combustion chamber or upstream of the             spraying inlet for introducing a residual solution Pr or a             mixture of residual solution Pr and of enriched phosphoric             acid solution P1 in sprayed form into a combustion unit,         -   the combustion unit being arranged in the upper part of the             combustion chamber, and being capable of forming a flame             having a temperature of at least 1500° C. by combustion of a             combustible, said combustion unit comprising:             -   a burner,             -   fluid connections between the burner and, on the one                 hand, an oxygen source and, on the other hand, a                 combustible source for feeding the flame,         -   a combustion outlet of the combustion chamber for recovering             a combustion solution P3 in liquid phase, and arranged             downstream of the combustion unit, which is itself arranged             downstream of the spraying and residue inlet,         -   an evacuation outlet for combustion gases G1 obtained from             the flame,     -   (B) a gas-acid contactor having         -   a feed inlet connected to a source of a feed solution F0,             for introducing at a contact feed flow rate Q0 a feed             solution F0,         -   a combustion gas inlet for introducing into the gas-acid             contactor combustion gases G1 at a flow rate Qg1,         -   a recirculation inlet which is identical to or different             from the contact feed inlet, for introducing a recirculated             enriched phosphoric acid solution P2 at a recirculation flow             rate Q2,         -   the feed and/or recirculation inlets and the gas inlet being             arranged to allow, on the one hand,             -   contact between the feed stream F0 and the recirculation                 stream F2 to form a stream of a mixture of the feed                 solution F0 and of the recirculated enriched phosphoric                 acid solution P2 and, on the other hand             -   contact of the mixture thus formed with the combustion                 gases G1,         -   one or more enriched phosphoric acid outlets,     -   (C) a combustion gas fluidic connection connecting an end         coupled to the combustion gas evacuation outlet of the         combustion chamber, to an end coupled to the combustion gas         inlet in the gas-acid contactor,     -   (D) a first spraying fluidic connection connecting an upstream         end (3 u) coupled         -   to the enriched phosphoric acid outlet of the gas-acid             contactor or         -   to a branching point with a first fluidic connection which             is coupled to the enriched phosphoric acid outlet,     -   to a downstream end coupled to the enriched phosphoric acid         inlet in the combustion chamber,         characterized in that the device also comprises     -   (E) a recirculation fluidic connection connecting an upstream         end coupled         -   to a recirculated enriched phosphoric acid outlet (1 pd) of             the gas-acid contactor or         -   to a branching point with the first fluidic connection,     -   to a downstream end coupled         -   to the recirculation inlet of the gas-acid contactor or         -   to a feed connection feeding the gas-acid contactor with             feed solution F0, and     -   (F) means for controlling and maintaining a ratio, Qp/(Qp+Q2),         between a spraying mass flow rate Qp flowing in the spraying         fluidic connection (3 p) and a total mass flow rate (Qp+Q2)         defined as the sum of the spraying mass flow rate Qp and of a         recirculation mass flow rate Q2 flowing in the recirculation         fluidic connection at a value of less than 50%, preferably less         than 10%, preferably less than 5%, more preferably less than         2.5% and in which the ratio Qp/(Qp+Q2) has a value of greater         than 0.1%, preferably greater than 0.5%.

The residue inlet is preferably in fluidic communication with a source of a residual solution Pr which is aqueous and comprises phosphorus molecules in orthophosphate and/or polyphosphate form and undesirable volatilizable materials.

BRIEF DESCRIPTION OF THE FIGURES

Various aspects of the present invention are illustration in the following figures.

FIG. 1: illustrates graphically the relationship between the boiling point and the P₂O₅ concentration at equilibrium of the liquid phase (part (a)) and of the vapor phase (part (b)), and also the relationship between the P₂O₅ concentration and the weight distribution of the phosphoric acid molecules according to the various values of n (part (c)).

FIG. 2: illustrate a device variant according to the present invention.

FIG. 3: reports values of an illustrative selection of parameters of the process according to the present invention.

FIG. 4: illustrates a device variant according to the present invention.

FIG. 5: illustrates a device variant according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2 to 5 illustrate the process and nonexhaustive variants of devices for forming said process. Hereinbelow, the term “stream” represented by the letter “F” is used in its commonly accepted interpretation as simply a flow of a fluid. Only FIG. 3 indicates the streams by the letter “F”. The other figures illustrating devices indicate flow rates “Q” corresponding to the streams “F” of FIG. 3. The term “flow rate” represented by the letter “Q” characterizes the mass of the stream per unit time and is expressed in [kg/s] or [kg/h]). The term “flow rate”, even used alone, thus defines a mass flow rate. In order to express the flow rates as a function of the nominal power of the combustion chamber, the mass flow rates of the various streams will be reported hereinbelow in units of [kg/(h MW)], which represents a flow rate per unit of nominal power [MW⁻¹] of the combustion chamber.

Since the combustion solution P3 derived from the combustion of the mixing solution Pm obtained at the end of the process of the present invention may comprise purified phosphoric acid alone or as a mixture with polymerized molecules whose respective concentrations vary according to the temperature and the P₂O₅ concentration of the mixing solution Pm contacting the flame of the combustion chamber (cf. FIG. 1), which itself depends inter alia on the concentration in the residual solution Pr used, reference will be made in the rest of the document to the solution P3 obtained by the expression “purified phosphoric acid solution”, even if it is clear that this solution may also comprise polymerized molecules and thus PPA, it may also contain impurities. It is also obvious that the degree of purity of the solution P3 will depend on the applications for which this purified acid is intended and that the presence of certain ions will not always be counter-indicated in certain applications.

Conventions and Definitions

Unless otherwise mentioned in the present patent, the term “concentration” is used to express mass concentrations (weight percentages, w/o). To define the content in the present case which is of principal concern, when reference is made to a concentration of the solutions of phosphoric acid or the like, this should be understood as the weight content expressed in P₂O₅ equivalent units, which will be written as “% eq. P₂O₅” or “% P₂O₅”. As regards the gas streams, for instance the combustion gases, where several species of interest may coexist as a function of the operating conditions, in gaseous or liquid form (for example by entrainment of droplets), or even optionally in solid form (fumes), the concentration in these streams is also expressed in P₂O₅ equivalent units (by weight, w/o). The ion dissociation of the species of interest is not considered in the present text. For information, the concentration of a phosphoric acid solution may also occasionally be expressed in H₃PO₄ equivalent units. The correspondence between the two concentration units is defined by the relationship: 1 eq. P₂O₅=0.7245 eq. H₃PO₄.

In the present text, the following expressions are understood as follows:

-   -   “phosphoric acid solution”, an aqueous solution comprising         HO[P(OH)(O)O]_(n)H, with n≥1     -   “orthophosphoric acid”, an aqueous solution very predominantly         comprising HO[P(OH)(O)O]_(n)H, with n=1, i.e. an aqueous         phosphoric acid solution containing less than 61% by weight of         P₂O₅;     -   “polyphosphoric acid solution” (=PPA), an aqueous solution         predominantly comprising HO[P(OH)(O)O]_(n)H, with n>1; i.e. an         aqueous phosphoric acid solution containing more than 76% by         weight of P₂O₅;     -   “polycondensation of polyphosphoric or orthophosphoric acid”,         the polycondensation of the molecules considered as represented         by equations (1) and/or (2) below;     -   “aqueous solution comprising phosphorus”, a solution containing         dissolved phosphorus in the form of species such as         orthophosphate or polyphosphates. Depending on the P₂O₅ content         of these solutions, the orthophosphate or polyphosphate species         may be present as presented in FIG. 1(c). These species may be         present in the form of ions.

Process—Gas-Acid Contacts

The process of the present invention comprises the introduction into a gas-acid contactor (1) of the following streams.

-   -   A feed stream F0 of feed solution P0 of the contactor at a feed         flow rate Q0. The feed solution P0 is an aqueous solution         preferably comprising phosphorus, preferentially in the form of         species such as orthophosphate (cf. FIG. 1) at a mass         concentration xp0 of between 0 and 54%, preferably from 0.1% to         50%, more preferably from 1% to 35%, preferably from 5% to 20%         P₂O₅. The feed solution F0 makes it possible to deliver into the         combustion chamber the species containing phosphorus which would         have left it with the combustion gases G1. In one variant of the         present invention, the solution P0 does not comprise any         phosphorus (xp0=0) and may be water. The solution P0 may contain         compounds which can react with the undesirable volatilizable         materials, to destroy them, for example aqueous hydrogen         peroxide solution, chlorate or nitrate ions. These compounds may         react, for example, with dissolved carbonaceous materials to         form carbon dioxide (CO₂) which is readily volatilizable.     -   In an alternative variant of the present invention, the solution         P0 is a source of phosphorus and comprises P₂O₅. For example,         the contact feed solution P0 may comprise a phosphorus         concentration xp0 of from 5% to 54%, preferably less than 45%,         more preferably between 10% and 35% P₂O₅. Higher concentrations         of the feed solution do not in any way hamper the process, and         make it possible to increase the phosphorus concentration xpm of         the mixing solution, Pm, reaching the flame in the combustion         chamber. The P₂O₅ content in the purified phosphoric acid         solution, P3, at the end of the process may thus be varied.     -   The flow rate Q0 of the solution P0 in the contactor expressed         in nominal power units [MW⁻¹] of the combustion chamber is         preferably between 100 and 3000 kg/(h/MW), preferably between         500 and 2500 kg/(h/MW), or else between 1000 and 2000 kg/(h MW).     -   In one variant of the present invention, the solution P0         comprises undesirable volatilizable materials typically such as         carbon, fluorine, chlorine, sulfur, nitrogen in soluble form         (ionic or nonionic). For example, they may be present in the         solution P0 in concentrations xpv of undesirable volatilizable         materials of a least 5 ppm (parts per million), or of at least         10 ppm, preferably of at least 100 ppm. Preferably, the         concentration xpv of undesirable volatilizable materials is less         than 5%, preferably less than 2% by weight of total organic         carbon relative to the total weight of the solution. For         example, the solution P0 may comprise at least 10 ppm of         fluorine, or at least 100 ppm of fluorine, at least 1% fluorine.         Depending on the applications, such solutions are unusable in         this form.     -   If the solution P0 does not comprise any undesirable         volatilizable materials, then a recirculation solution Pr         containing such undesirable volatilizable materials is added to         the combustion chamber in order to produce a mixing solution         comprising undesirable volatilizable materials in the         concentrations indicated above.     -   To avoid excessive deposition of soiling in the gas-acid         contactor, it is preferable for the majority or all of the         undesirable volatilizable materials to be introduced directly         into the combustion chamber, contained in the residual solution         Pr.     -   A recirculation stream F2 of recirculated enriched phosphoric         acid solution P2 which will be defined in greater detail         hereinbelow. The flow rate of the recirculated enriched         phosphoric acid solution P2 in the contactor expressed in         nominal power units [MW⁻¹] of the combustion chamber may be         between 300 and 120 000 kg/(h MW), preferably between 600 and         100 000 kg/(h MW), preferably greater than 9000 kg/(h MW),         preferably between 1500 and 80 000 kg/(h MW). The recirculation         stream F2 of recirculated enriched phosphoric acid solution P2         is the fruit of the contact of a mixture of the feed stream F0         and the recirculation stream F2 of the recirculated enriched         phosphoric acid solution P2 from a previous cycle with a         combustion gas stream G1.     -   A stream of combustion gases G1 which are formed during the         combustion of a mixing stream Fm of a mixing solution Pm         comprising phosphorus at a mass concentration xpm higher than         that of the contact feed stream F0 in a combustion chamber which         will be described and discussed in detail hereinbelow. The         combustion gas stream G1 comprises phosphorus molecules in the         form of droplets or of vapors, carried from the combustion         chamber to the gas-acid contactor. For example, the combustion         gas stream G1 may comprise between 0.1% and 15% P₂O₅, for         example between 0.5% and 13%, or else between 1% and 10% P₂O₅,         preferably between 2% and 5% P₂O₅. Furthermore, the combustion         gases comprise undesirable volatilized materials which are         derived from the combustion of the mixing solution Pm.

The feed stream F0 and the recirculation stream F2 and the combustion gases G1 are thus contacted together in the gas-acid contactor (this is thus referred to as a direct gas-acid contactor) to form, on the one hand, an enriched phosphoric acid solution P1 and, on the other hand, contacted combustion gases G3.

In a preferred variant, illustrated in FIGS. 2 and 4, the feed stream F0 and the recirculation stream F2 are mixed before being introduced into the gas-acid contactor to form a mixing stream of the feed solution P0, of the contactor and of the recirculated enriched phosphoric acid solution P2. To do this, it suffices to branch a recirculation pipe (3 r) conveying the stream F2 and a feed pipe (3 a) conveying the stream F0 upstream of an inlet (1 pu) of the gas-acid contactor (1). The pressures in the recirculation pipe (3 r) and the feed pipe (3 a) must be controlled so as to avoid reflux of liquid in one of the two branched pipes. FIG. 2 shows a feed pipe (3 a) branched in the recirculation pipe (3 r), whereas FIG. 4 illustrates a recirculation pipe (3 r) branched in a feed pipe (3 a). In the two configurations, the combustion gases G1 are then placed in contact with the mixture of solution streams (F0+F2) thus formed, after introducing said mixture into the gas-acid contactor (1).

In an alternative variant, illustrated in FIGS. 3 and 5, the feed stream F0 and recirculation stream F2 are contacted after having been introduced separately into the gas-acid contactor to form a stream of a mixture of the feed solution P0 of the contactor and of the recirculated enriched phosphoric acid solution P2. The phosphoric acid stream F0 and F2 and the combustion gas stream G1 are thus all placed in contact in the gas-acid contactor. It suffices to provide in the gas-acid contactor an inlet (1 pu) for the feed stream F0 separate from an inlet (1 pru) for the recirculation stream F2.

The contact between the feed stream F0 and the recirculation stream F2 or their mixture (F0+F2) and the combustion gases G1 in the gas-acid contactor may take place by co-current or counter-current stream contact. In particular, the liquid phases flow downward in the direction of gravity, and the gaseous phase rises upward. In a preferred variant, the two or three streams flow co-currentwise from an upper part to a lower part of the gas-acid contactor. In the context of the present invention, the terms “upper” and “lower” are understood according to the direction of the forces of Earth's gravity which extend in the direction of the center of gravity of the Earth. Thus, in the absence of pressure gradients, a liquid naturally flows from the upper part of a reactor to its lower part which is downstream of the upper part following the direction of the Earth's gravity.

It is possible to contact the combustion gases G1 with the phosphoric acid feed and recirculation streams F0&F2 or their mixture (F0+F2) by guiding the combustion gases in a transverse stream relative to that of the phosphoric acids. The placing in contact in co-current streams is, however, preferred.

The contact between the feed stream F0 and the recirculation stream F2 or their mixture (F0+F2) and the combustion gases G1 forms an enriched phosphoric acid solution P1 and contacted combustion gases G3. This contact may take place by percolating the streams through a packing material which withstands the operating conditions. During the contact between the combustion gas G1 and the feed stream F0 and recirculation stream F2, exchanges take place. On the one hand, the phosphorus molecules transported by the combustion gases in the form of droplets and vapor are carried by the streams F0 and F2, allowing the formation of the enriched phosphoric acid solution P1 having a P₂O₅ content higher than that of each of the streams F0 and F2. On the other hand, an exchange of heat takes place between the hot combustion gases, at a temperature Tg1 between 200 and 600° C., toward the aqueous solutions of the streams F0 and F2 which are at lower temperatures, as indicated in FIG. 3. The contacted gases G3 are thus at a temperature Tg3<Tg1 facilitating their subsequent treatments for the purpose of evacuating them into the atmosphere. At the same time, the enriched phosphoric acid solution P1 is thus at a temperature T1 above that of the mixture of the solutions P0 and P2; T1 is higher than the temperature T0 of the feed solution P0 (which may be of the order of 20 to 200° C.) and is substantially equal to the temperature T2 of the recirculated enriched phosphoric acid solution P2 since it is the same solution at two ends of the recirculation loop (3 r).

The enriched phosphoric acid solution P1 and the contacted combustion gases G3 formed following the placing in contact of the phosphoric acid feed and recirculation streams F0&F2 with the combustion gases G1 are then separated by separation means that are well known to those skilled in the art, such as a centrifugal separator or by gravity, a coalescer, a spray eliminator, a mattress, chicanes, etc. The contacted combustion gases G3 are then evacuated from the gas-acid contactor (1) at a temperature substantially below that of the contacted combustion gases G3 introduced into said gas-acid contactor for subsequent treatments. As the majority of the phosphorus molecules contained in the combustion gas stream G1 are transferred into the stream F1 of enriched phosphoric acid solution P1 during the contact of the stream G1 with the streams F0 and F2, the contacted gas stream G3 is much more depleted in P₂O₅ than the combustion gas stream G1 with contents which may be less than 1% P₂O₅.

The contacted combustion gases G3 containing the undesirable volatilized materials may also undergo scrubbing after they have left the gas-acid contactor, with an aqueous scrubbing solution in order to dissolve and remove, before releasing the gases into the atmosphere, the undesirable compounds, for instance fluorinated or chlorinated compounds, SO₃, etc. Other treatments of the contacted combustion gases G3 are possible, for example including condensation of the gases in an indirect condenser. The enriched phosphoric acid solution P1 is also removed from the gas-acid contactor separately of the contacted combustion gases G3.

Process—Stream F1 and Division Into Streams Fp and F2

The enriched phosphoric acid solution P1 may comprise a concentration xp1 of P₂O₅ units of greater than or equal to 1%, preferably less than 60%, more preferably between 5% and 50%, preferentially between 10% and 40% P₂O₅. The P₂O₅ concentration of the enriched phosphoric acid solution P1 obviously depends on the P₂O₅ concentration of the feed solution, P0, and on the combustion gas stream, G1. As discussed hereinbelow, the phosphorus concentration of the enriched phosphoric acid solution P1 is generally higher than that of the feed solution, P0.

Before, during or after its evacuation from the gas-acid contactor, the enriched phosphoric acid solution P1 is divided into two separate streams:

-   -   a recirculation stream F2 of recirculated enriched phosphoric         acid solution P2 to introduce it into the gas-acid contactor (1)         through a recirculation loop (3 r) to place it in contact with         the feed stream F0 and the combustion gases G1 as described         hereinabove, and     -   a spraying stream Fp of a spraying solution Pp to introduce it         into a combustion chamber (2).

The spraying solution Pp and the recirculated enriched phosphoric acid solution P2 are identical to each other in composition and identical to the enriched phosphoric acid solution P1 (P1=Pp=P2) since they have not undergone any alteration between the moment of their formation in the gas-acid contactor and the division into two separate streams, the recirculation stream F2 and the spraying stream Fp. The temperatures Tp and T2 of the solutions Pp and P2 are also substantially identical to the temperature T1 of the solution P1 which may be of the order of 100 to 300° C. The solutions Pp and P2 preferably comprise, in the stationary regime, a higher P₂O₅ concentration than that of the feed solution P0 of the contactor. For example, the solutions Pp and P2 may comprise phosphorus concentrations of between 1 and 60%, preferentially between 5% and 50% P₂O₅, preferentially between 10% and 40% P₂O₅. This is explained by two main reasons.

Firstly, the placing in contact of the phosphoric acid feed and recirculation streams with the combustion gases G1 which are at a higher temperature Tg1 of the order of 300 to 600° C. (cf. FIG. 3), entrains the evaporation of part of the water contained in the aqueous phase of the solutions P0 and P2, which de facto increases the P₂O₅ concentration of the solutions P1, Pp and P2.

Secondly, as shall be discussed hereinbelow, the combustion gases G1 formed during the combustion of the mixing stream Fm of mixing solution Pm in the combustion chamber comprise phosphoric acid droplets or vapors. The combustion gases G1 may comprise between 0.1% and 15% P₂O₅, preferably between 0.5% and 13%, preferably between 1% and 10%, preferably between 2% and 5% (cf. FIG. 3). During the contact with the feed streams of solution P0 and of circulated enriched phosphoric acid P2, the majority of these molecules are transferred from the combustion gases to the mixture of acidic solutions (P0+P2). After contact, the contacted combustion gases G3 contain a much smaller amount of P₂O₅ molecules than the gas G1 before contact, generally less than 1%; preferably less than 0.5%, advantageously less than 0.1% by weight (cf. FIG. 3). By means of this transfer of P₂O₅ molecules to the mixture of acids, the P₂O₅ concentration of said mixture increases.

In one variant of the invention, the enriched phosphoric acid solution P1 is divided into two streams, the spraying stream Fp and the recirculation stream F2 at the outlet of the gas-acid contactor in a spraying fluidic connection (3 p) and in a recirculation fluidic connection (3 r), respectively, as illustrated in FIG. 2. Each of the fluidic connections (3 p) and (3 r) is equipped with a pumping system (4, 4 r) to ensure the flow rates and a spraying stream Fp at a spraying flow rate Qp toward a combustion chamber (2) and to entrain the recirculation stream F2 at a flow rate Q2 toward the gas-acid contactor, thus forming a recirculation loop.

In an alternative variant, the enriched phosphoric acid solution P1 is removed from the gas-acid contactor in a first fluidic connection (3, 3 u) which is common and which divides into two at a T-shaped or Y-shaped branching point (5) with, on the one hand, the spraying fluidic connection (3 p) which entrains the spraying stream Fp at a flow rate Qp, toward a combustion chamber (2) and, on the other hand, a recirculation fluidic connection (3 r) which entrains the recirculation stream F2 at a flow rate Q2 toward the gas-acid contactor, thus forming a recirculation loop. Different variants of this configuration comprising a branching point (5) are illustrated in FIGS. 3 to 5. The spraying flow rate Qp and recirculation flow rate Q2 may be ensured by one or more valves (cf. FIGS. 3 and 4), by pumps (4, 4 r) on each of the branches of the branching point (5) (cf. FIG. 5) and/or by sections of spraying pipes (3 p) and of recirculation pipes (3 r) dimensioned to obtain the desired flow rates or other well-known means that are used industrially for dividing a stream between two feeds (for example T-shaped or Y-shaped pipes with sets of regulated valves).

The enriched phosphoric acid solution P1 leaves the contactor at a total flow rate Q1=(Qp+Q2). The total flow rate Q1 expressed in nominal power units [MW⁻¹] of the combustion chamber is preferably between 600 and 123 000 kg/(h MW) or between 1000 and 120 000 kg/(h MW), preferably between 12 000 and 100 000 kg/(h MW). As discussed hereinabove, the stream F1 of enriched phosphoric acid (P1) is divided into two streams Fp and F2 each having a spraying flow rate Qp and a recirculation flow rate Q2. The division into two streams may take place before leaving the gas-acid contactor, at the outlet thereof, or after the outlet. The spraying flow rate Qp and recirculation flow rate Q2 must be determined as a function, inter alia, of the capacity of the combustion chamber and of the gas-acid contactor, of the temperature of the combustion gases G1 and of their P₂O₅ content.

In a stationary production state, the ratio, Qp/(Qp+Q2), between the mass flow rate Qp of the spraying stream Fp and the total mass flow rate (Qp+Q2) of the enriched phosphoric acid stream F1 (which is in fact the sum of the spraying flow rate Qp and the recirculation flow rate Q2) is preferably less than 50%, preferably less than 20% and more preferably less than 10%. In a preferred variant of the invention, the ratio Qp/(Qp+Q2) is less than 5%, preferably less than 4%, more preferably less than 2.5% and even less than 2%. The ratio Qp/(Qp+Q2) is preferably greater than 0.1%, or else greater than 0.2% and preferably greater than 0.5%. Increasing the flow rate Q2 relative to the flow rate Qp makes it possible, on the one hand, to cool the combustion gases G1 to a lower temperature, which is necessary before their evacuation, and, on the other hand, to further enrich the P₂O₅ content of the enriched phosphoric acid solution Pp.

The ratio, Q2/(Qp+Q2), between the mass flow rate Q2 of the recirculation stream F2 and the total mass flow rate (Qp+Q2) is, needless to say, the remainder of the ratio Qp/(Qp+Q2), the sum of which is equal to 100%. The recirculation flow rate Q2 is thus preferably greater than or equal to the spraying flow rate Qp and, in certain preferred variants, is considerably higher than Qp with a ratio of the flow rates, Qp/Q2, which may range from 0.1/99.9 to 49/51 (=0.1% to 96%). Preferably, the flow rate ratio Qp/Q2 is between 1/99 and 5/95 (=1% to 5.3%). Preferably, the flow rate ratio Qp/Q2 is between 1/99 and 4/95 (=1% to 4.2%).

As discussed hereinabove, the flow rate Q2 of the recirculated enriched phosphoric acid solution P2 in the contactor expressed in nominal power units [MW⁻¹] of the combustion chamber may be between 300 and 120 000 kg/(h MW), preferably between 600 and 110 000 kg/(h MW), preferably between 9000 and 100 000 kg/(h MW). Thus, the flow rate Qp of the spraying solution Pp flowing toward the combustion chamber expressed in nominal power units [MW⁻¹] of the combustion chamber may be between 300 and 3000 kg/(h MW), preferably between 600 and 2000 kg/(h MW), preferably between 1000 and 1500 kg/(h MW).

Process—Streams Fp, Fr and Fm

A mixing stream Fm of a mixing solution Pm comprising undesirable volatilizable materials at a non-zero mass concentration xpv and phosphorus at a mass concentration xpm preferably higher than that of the contact feed stream F0 is formed by the spraying stream Fp, optionally mixed with a residual stream Fr of an aqueous residual solution Pr originating from the residues of a prior industrial process. If the feed solution F0 does not comprise any undesirable volatilizable materials, then the mixing of the feed stream P0 with a residual stream Fr is obligatory. Otherwise, it is optional, but preferred. The mixing stream Fm is sprayed through a flame burning in the upper part of a combustion chamber (2) to:

-   -   optionally oxidize the undesirable volatilizable materials and         to volatilize them,     -   form combustion gases G1 carrying the undesirable volatilizable         materials thus volatilized (=undesirable volatilized materials)         and thus purify the residual solution Pr,     -   evaporate water and thus concentrate the mixing solution Pm,     -   form a purified phosphoric acid solution P3,     -   optionally polymerize, depending on the initial P₂O₅ content,         purified phosphoric acid molecules as polyphosphoric acid.

The residual stream Fr comprises:

-   -   a mass concentration, xpr, of at least 1% P₂O₅,     -   a mass concentration, xer, of water, and     -   undesirable volatilizable materials.

The residual stream Fr comprises a phosphorus concentration xpr of at least 1% or of at least 5%, preferably at least 10%, more preferably at least 15%, preferably at least 20% P₂O₅. The flow rate Qr of the residual solution Pr in the combustion chamber expressed in nominal power units [MW⁻¹] of the combustion chamber is preferably non-zero and preferably between 5 and 1500 kg/(h MW), preferably between 400 and 1000 kg/(h MW). The temperature of the residual solution Fr may be between 20 and 200° C., preferably between 40 and 150° C., more preferably between 50 and 100° C. Preheating of the solution Fr is advantageous in terms of efficiency of combustion of the mixing solution Fm in the flame.

The residual solution Pr and, optionally, the feed solution P0 is or preferably contains a solution coming from industry. This solution may be generated by the scrubbing of facilities or during common production or maintenance operations in industries such as the metallurgical, agrifood, pharmaceutical or chemical industries and particularly during the production of phosphate salts or of fertilizers. These solutions are generally difficult to recycle in their native form on account of their contents of various pollutants, notably soluble residues of organic materials and due to their low phosphorus concentrations. They must thus be treated before being subsequently concentrated. The residual solution may also be derived from processes for recovering phosphorus from starting materials known as “secondary” materials and which are in fact solid compounds containing phosphorus other than phosphate ore. Mention may notably be made of bone powder or bone powder ash, sludges or sludge ash from purification stations, pig and poultry manure or manure ash, etc. These residual solutions or residual acid solutions or residual solutions comprise P₂O₅ but also often undesirable volatilizable materials typically such as carbon, fluorine, chlorine, sulfur, nitrogen in soluble form (ionic or nonionic). The concentrations of the undesirable volatilizable materials obviously depend on the origin of the residual solution. For example, they may be present in the residual solution Fr in concentrations xpv of undesirable volatilizable materials of at least 5 ppm (parts per million), preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, more preferably of at least 5% by weight of total organic carbon relative to the total weight of the solution or else at least 10 ppm of fluorine, at least 100 ppm of fluorine, at least 1% of fluorine. Depending on the applications, these solutions are unusable as such.

In a stationary production state, the ratio, Qp/Q0, between the flow rate Qp of the spraying stream Fp and the flow rate Q0 of the feed stream F0 is preferably between 100% and 250%, preferably between 101% and 140%, preferably between 110% and 115%. This ratio may be higher than 100% since the contact of the streams Q0 and Q2 with the combustion gases in the gas-liquid contactor increases the mass of the stream F1 leaving the gas-liquid contactor. The value of this ratio may decrease as the value of the residual flow rate, Qr, increases.

The flow rate ratio Qr/(Qp+Qr) between the residual flow rate Qr and the sum of the spraying flow rate Qp and the residual flow rate Qr represents the flow rate fraction of the residual solution Pr entering the combustion chamber. The value of this ratio depends inter alia on contents of P₂O₅ and of undesirable volatilizable materials of the residual solution and/or of the feed solution, which are determining for the content of P₂O₅ and of undesirable volatilizable materials of the mixing solution Pm. For example, the ratio Qr/(Qp+Qr) may be between 0 and 94%, preferably between 5% and 90%, more preferably between 10% and 80% or else between 15% and 45%.

Independently of the value of the ratio Qr/(Qp+Qr), the mixing solution Pm preferably comprises a phosphorus concentration xpm of greater than 1%, preferably greater than 2% or 5%, more preferably greater than 20%, more preferably greater than 30%, preferably greater than 40%, and more preferably between 45% and 60% P₂O₅. The flow rate Qm of the solution Pm in the combustion chamber is the sum of the spraying flow rate Qp and of the flow rate of the residual solution Qr. Expressed in nominal power units [MW⁻¹] of the combustion chamber, the mixing flow rate Qm is preferably between 305 and 3000 kg/(h MW), preferably between 900 and 2000 kg/(h MW).

Process—Evaporation of the Undesirable Volatilizable Materials and Concentration of P₂O₅

A main function of the combustion chamber is to degrade, if necessary, by oxidation and then to vaporize the undesirable volatilizable materials present in the residual solution. A second function of the combustion chamber is to evaporate the water present in the solutions to concentrate the residual and feed solutions. A third (optional) function is polycondensation of the phosphate molecules present into polyphosphoric acid (PPA). The distribution of the species present in the solution thus formed depends on the P₂O₅ concentration of the mixing solution Fm reaching the combustion flame, and also on the polycondensation temperature. As may be seen in FIG. 1(c), PPA forms only if the P₂O₅ content is sufficiently high, about 60% P₂O₅, which is higher than the phosphorus content generally present in residual solutions Fr. If PPA is desired, it is then necessary to increase the phosphorus content of the mixing solution Fm by feeding the combustion chamber with a spraying solution Fp having a higher phosphorus concentration or with a residual solution Fr having a higher phosphorus concentration.

Combustion in the flame of the mixing solution Pm thus forms, on the one hand, combustion gases G1 formed by the evaporation of the water and, in particular, of the undesirable volatilizable materials and, on the other hand, a combustion solution P3 which is in liquid form and comprising phosphorus and, if the P₂O₅ concentration and the polycondensation temperature Tpc are sufficient, species polymerized by polycondensation of the phosphoric acid contained in the solution Pm.

The temperature reached by the mixing solution Pm in the flame is an important parameter of the process since it will enable the volatilization of the undesirable volatilizable compounds present in the residual solution and thus in the mixing solution. The P₂O₅ concentration obtained in the combustion solution P3 is also dependent thereon, as shown by the graph of FIG. 1(a). It is also important to maintain the mixing solution containing phosphoric acid in contact with the flame and the combustion gases for a time that is sufficient for the water to be able to evaporate and optionally for the polycondensation to be able to take place.

The flame is fed with a combustible and a source of oxygen, typically air or, for a higher temperature, oxygen. The flame is preferably a slightly oxidizing flame, preferably comprising between 1% to 5% of excess air. The combustible is preferably natural gas, butane, propane or any other combustible, whether it is gaseous or liquid. In the absence of spraying of the mixing solution Pm, the flame preferably reaches a theoretical temperature of at least 750° C., preferably at least 1000° C., more preferably at least 1700° C., for example 1800° C.±50° C. In the process of the present invention, the temperature increase is instantaneously limited since, on the one hand, the mixing solution Pm is fed at a lower temperature Tm, of the order of 20-300° C. and, on the other hand, because the evaporation of the water molecules from the solution is energy-intensive.

The residual stream Fr and the spraying stream Fp may be mixed to form the mixing stream Fm before being sprayed into the flame in the combustion chamber, as illustrated in FIGS. 2 and 5. Alternatively, the two streams Fr and Fp may be sprayed separately into the combustion chamber to form the mixing stream Fm in the flame or just before reaching the flame, as illustrated in FIGS. 3 and 4.

The combustion solution P3 which consists of a purified phosphoric acid solution is thus an aqueous phosphoric acid solution which may contain polymerized species depending on the P₂O₅ content present in the solution (cf. FIG. 1).

If the production of PPA is desired, it is preferable for the mixing solution sprayed into the flame to reach a polycondensation temperature Tpc of at least 400° C., preferably at least 500° C. and even higher than 550° C., of the order of 650° C. or 700° C., for a predefined polycondensation time. A high polycondensation temperature Tpc makes it possible to obtain polyphosphoric acid solutions with a high P₂O₅ concentration, of the order of 86% and more, with longer chain lengths n (e.g. n≥5 to 12) (cf. FIG. 1(c)). The temperatures required for the polycondensation of phosphoric acid require chemically and thermally resistant materials for the various elements of the reaction device. The mixing solution Pm, which comprises orthophosphoric acid molecules and optionally polyphosphoric acid oligomers (of m+1 condensed units) undergoes a polycondensation reaction under the action of the temperature to release water and to form longer polymer chains, according to equation (1) described hereinabove and according to equation (2) (with m≥1 and r≥1):

The combustion solution P3 thus formed is then separated from the combustion gases G1 formed during the evaporation of water and from the undesirable volatilizable materials and optionally the polycondensation of polyphosphoric acid in a gas-liquid separator (9). The combustion solution P3 containing phosphoric acid and optionally polyphosphoric acid and virtually free of undesirable volatilizable materials is recovered whereas the combustion gases G1 are transferred into the gas-acid contactor (1) to be placed in contact with the feed stream F0 and the recirculation stream F2, as described hereinabove.

The stream F3 of the combustion solution P3 thus recovered may have a high temperature of the order of 150 to 700° C., preferably 200 to 650° C., preferably from 300 to 500° C. depending on the vaporization temperature of the undesirable volatile materials contained in the mixing solution Pm. Specifically, the temperature required for volatilization of the volatilizable materials present in the mixing solution Pm varies according to the nature of the materials present in the mixing solution Pm. It is preferable to cool the solution P3 in a heat exchanger (11) (cf. FIG. 2) to a temperature below T3, which allows a wider choice of materials for the tank for storing phosphoric acid (and optionally purified polyphosphoric acid) thus formed and cooled, while at the same time maintaining the solution in liquid form.

The combustion solution P3 comprising purified phosphoric acid (and optionally polyphosphoric acid) thus formed and recovered comprises a concentration of undesirable volatilizable materials lower than that of the mixing solution Pm. For example, the combustion solution Pm comprises less than 50% of the undesirable volatilizable materials contained in the residual solution Pr, preferably less than 70%, more preferably less than 80% or less than 90%, and ideally less than 95% or 99%. The combustion solution P3 comprises a higher P₂O₅ concentration than that of the mixing solution Pm. This is explained by the evaporation of a majority of the water from the solution during its passage into the flame. The P₂O₅ concentration xp3 of the combustion solution P3 is normally greater than 10% P₂O₅, preferably greater than 15%, preferably greater than 25%, particularly preferably greater than 40%, or is preferably between 30% and 76%.

The flow rate Q3 of the combustion solution P3 out of the combustion chamber is representative of the phosphoric acid purification capacity. Expressed in nominal power units [MW⁻¹] of the combustion chamber, the flow rate Q3 is preferably between 240 and 1500 kg/(h MW), preferably between 500 and 1000 kg/(h MW).

The combustion gases G1 consist mainly, on the one hand, of CO₂, O₂, H₂O, and, on the other hand, of undesirable volatilizable materials, for instance nitrogen oxides (NOx), sulfur oxides, fluorinated or chlorinated compounds or organic materials, and also molecules containing phosphorus, the latter possibly being presents in amounts which may range between 0.1% and 15% by weight of P₂O₅, depending on the concentration xpm of the mixing solution Pm. In general, the P₂O₅ content that may be present in the combustion gases G1 ranges between 0.5% and 13% by weight, preferably between 1% and 10%, preferably between 2% and 5% P₂O₅. The temperature Tg1 of the combustion gases G1 transferred is substantially lower than the temperature which may be reached by the flame since, as discussed hereinabove, the temperature in the combustion unit falls during the polycondensation reaction which requires a substantial amount of energy, mainly to evaporate the water of the polycondensation reaction. The combustion gases enter the acid-gas contactor at a temperature Tg1 which is of the order of the polycondensation temperature Tpc, and is generally between 200 and 600° C., preferably between 400 and 500° C.

Process—Recirculation Loop and Combustion Gas

As discussed above, a recirculation fraction of the enriched phosphoric acid solution P1 leaving the gas-acid contactor (1) is reintroduced into the gas-acid contactor thus forming a recirculation loop, whereas a spraying fraction Pp is conveyed to the combustion chamber (2). The recirculation fraction is preferably greater than or equal to the spraying fraction and is ideally considerable greater than the spraying fraction, with ratios Qp/Q2 of the spraying flow rate Qp to the recirculation flow rate Q2 which may range from 0.1/99.9 to 49/51 (=0.1% to 96%). Preferably, the flow rate ratio Qp/Q2 is between 1/99 and 5/95 (=1% to 5.3%).

During their introduction into the gas-acid contactor, the feed stream F0 and the recirculation stream F2 may be mixed before being introduced into the gas-acid contactor to form a stream of a mixture of the contact feed solution P0 and of the recirculated enriched phosphoric acid solution P2, as illustrated in FIGS. 2 and 4. Alternatively, the streams F0 and F2 may be contacted after having been introduced separately into the gas-acid contactor to form a stream of a mixture of the contact feed solution P0 and of the recirculated enriched phosphoric acid solution P2, as illustrated in FIGS. 3 and 5.

The recirculation loop is an important element of the present invention. The main consequence of introducing such a recirculation loop is that the ratio (Qg1/(Q0+Q2)) between the mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor (1) and the total mass flow rate (Q0+Q2) of the feed stream F0 and the recirculation stream F2 introduced into the gas-acid contactor (1), is much lower than in the absence of such a recirculation loop. The ratio (Qg1/(Q0+Q2)) according to the present invention is preferably between 0.1% and 50%, more preferably between 0.5% and 20% or less than 10%, and is ideally between 1% and 7%. In the absence of such a recirculation loop (i.e. Qg1>0, Q0>0 and Q2=0), the ratio (Qg1/Q0) is considerably greater, with values of greater than 60%, in general greater than 100%, indicative of a combustion gas flow rate Qg1 which is higher than the feed flow rate Q0 of feed solution P0 of the contactor.

The recirculation loop thus makes it possible to control the ratio between the flow rate of combustion gas G1 and the total flow rate (Q0+Q2) of phosphoric acid feed solutions P0 and of recirculated enriched phosphoric acid solution P2. In particular, it makes it possible to considerably increase the mass of phosphoric acid solution contacted with the combustion gas. This has several advantages.

On the one hand, the transfer of the P₂O₅ droplets and vapors contained in the combustion gases G1 to the stream of the mixture of solutions P0 and P2 is much more pronounced. The P₂O₅ concentration of the spraying solution formed during contact with the combustion gases is thus higher than if the flow rate ratio Qg1/(Q0+Q2) had been higher. The better gas/liquid contact thus obtained allows better recovery by the enriched phosphoric acid solution P1 of the P₂O₅ contained in the combustion gases G1. Furthermore, the combustion gases G3 after contact with the streams F0 and F2 are thus purified of their P₂O₅ content, lightening their treatment before they are released into the atmosphere.

On the other hand, with such flow rate ratios, the temperature Tg3 of the combustion gases G3 after their contact with the streams F0 and F2 is reduced much more efficiently than in the process described in EP2411325 B1, thus not requiring any other heat exchanger (or at least a heat exchanger of lower capacity), which is essential in the process of EP2411325 B1 to lower the temperature of the combustion gases to a value that is acceptable for their evacuation into the atmosphere.

Device

The process of the present invention may be implemented in a device comprising a combustion chamber (2), a gas-acid contactor (1) and various fluidic connections between the combustion chamber and the gas-acid contactor. It is clear that the device may comprise several combustion chambers and/or several gas-acid contactors positioned in parallel or in series.

Device—Combustion Chamber (2)

The combustion chamber (2) makes it possible to perform the combustion of the mixing solution Pm, by spraying it into the flame. The mixing solution Pm is formed from the spraying solution Pp mixed with the residual solution Pr to form a combustion solution P3 comprising phosphoric acid (and possibly polyphosphoric acid) purified of the undesirable volatilizable materials. The walls of the combustion chamber must withstand the corrosive nature of the spraying solution Pp and of the residual solution Pr and the high temperatures prevailing therein. It is preferable for the walls to be made of silicon carbide or of amorphous carbon. It is possible to use jackets with a neutral gas or the combustion gases circulating between the two walls, which may have advantages in terms of temperature of the walls, and impermeability of said walls to the (poly)phosphoric acid solutions.

The combustion chamber (2) has one or more spraying inlets (2 pu) into the combustion chamber for introducing a spraying solution Pp at a flow rate Qp, or a mixing solution Pm at a flow rate (Qp+Qr), in sprayed form into a combustion unit located in an upper part of the combustion chamber (cf. FIGS. 2 and 4). In one variant of the invention, the combustion chamber may comprise one or more residual inlets (2 pdu) for introducing a residual solution Pr at a flow rate Qr, separate from the spraying inlet(s) (2 pu) (cf. FIGS. 3 and 5). A feed of an inert gas, such as nitrogen, may be provided to optimize the spraying of the spraying solution Pp and of the residual solution Pr and/or of the mixing solution Pm, which may have a high viscosity at the inlet of the combustion chamber. The residue inlet (2 pdu) is in fluidic communication with a source of a residual solution (Pr) which is aqueous and comprises phosphorus and undesirable volatilizable materials.

The combustion chamber (2) comprises a combustion unit (2 c) arranged in the upper part of the combustion chamber, and capable of forming a flame having a temperature of at least 1000° C., preferably at least 1500° C., and even at least 1700° C., preferably 1800° C.±50° C., by combustion of a combustible in the presence of oxygen. The temperature of the flame may be controlled by varying the flow rate of oxygen feeding the flame. The combustion unit comprises:

-   -   a burner,     -   fluidic connections between the burner and, on the one hand, a         source of oxygen and, on the other hand, a source of combustible         (10) for feeding the flame. Control of the ratio between the         supplies of combustible and of oxygen to the burner make it         possible to control the temperature of the flame. Preferably,         the combustible used is chosen from natural gas, methane,         butane, propane. The source of oxygen is generally air or         oxygen.

The combustion chamber (2) is equipped with a gas-liquid separator (9) to separate the combustion solution P3 thus formed from the combustion gases G1. For example, the combustion gases may be separated from the combustion solution by enlarging the flow rate transverse surface area, the consequence of which is to reduce the flow speed and thus the kinetic energy of the gas stream G1 and the combustion stream F3. As the streams flow from the top downward, by lowering their kinetic energy, the gases will slow down and can be diverted toward a deflector which guides them to the combustion gas outlet. By means of their higher density, the purified phosphoric acid and optionally polyphosphoric acid droplets of the combustion solution P3 continue their flow downward by gravity.

The combustion chamber (2) has a combustion outlet (2 pd) from the combustion chamber for recovering a purified (poly)phosphoric acid liquid phase, and arranged downstream of the combustion unit, which is itself arranged downstream of the mixing or spraying, and residue inlet. The term “downstream” is expressed relative to the direction of flow of the spraying solution Pp and the polyphosphoric acid solution P3 in the combustion chamber. As explained hereinabove, the direction of flow is preferably from the top downward following the direction of gravity. The device may thus be equipped with a tank for storing the phosphoric acid thus produced (not illustrated). Preferably, the device comprises a heat exchanger (11) arranged between the combustion outlet (2 pd) and the storage tank, in order to cool the combustion solution P3 from a temperature of between about 200 and 650° C. to a temperature of the order of 100 to 150° C. when it reaches the storage tank.

Finally, the combustion chamber (2) is equipped with an outlet for evacuating the combustion gas G1 obtained from the flame. These combustion gases are charged with droplets and vapors of P₂O₅ and of undesirable volatilized materials. They have a temperature Tg1 and do not need to be cooled before being introduced into the gas-acid contactor.

Device—Gas-Acid Contactor (2)

The gas-acid contactor (1) makes it possible to heat and to increase the equivalent concentration of P₂O₅ units of the feed solution introduced into the contactor, before it enters the combustion chamber (2) so as to optimize the phosphoric acid purification yield and the energy consumption of the polycondensation reaction.

The gas-acid contactor (1) has a feed inlet (1 pu) connected to a source of a feed solution P0 of the contactor or of a mixture of feed solution P0 of the contactor and of enriched phosphoric acid solution P2. As discussed hereinabove, the feed solution P0 of the contactor comprises between 0 and 54% P₂O₅, preferably from 0.1% to 50%, preferably from 1% to 35%, more preferably between 15% and 20% P₂O₅. The feed inlet (1 pu) must be dimensioned to allow the introduction of the feed solution P0 of the contactor at a feed flow rate Q0 or the introduction of the mixture of feed solution P0 of the contactor and of recirculated enriched phosphoric acid solution P2 at a flow rate (Q0+Q2). The recirculated enriched phosphoric acid solution, P2, may also be introduced into an inlet (1 pru) for recirculated enriched phosphoric acid, P2, separate from the feed inlet (1 pu).

The gas-acid contactor (1) is preferably a direct contactor. It comprises a combustion gas inlet (1 gu) for introducing into the gas-acid contactor combustion gases G1 coming from the outlet for evacuating the combustion gases G1. The feed inlet (1 gu) must be dimensioned to allow the introduction of the combustion gases G1 at a flow rate Qg1. As discussed hereinabove, the combustion gases G1 placed in contact with the feed solution P0 of the contactor make it possible (a) to increase the temperature of the feed solution P0 of the contactor, (b) to evaporate part of the water from the feed solution P0 of the contactor and (c) to exchange with the solution P0 the P₂O₅ droplets and vapors contained in the combustion gas G1.

The gas-acid contactor (1) is equipped with a recirculation inlet (1 pru), for introducing a solution of recirculated enriched phosphoric acid P2. In one variant of the invention, the streams F0 and F2 are mixed before being introduced into the gas-acid contactor and the recirculation inlet is then the same as the feed inlet (1 pu). In an alternative variant, the feed inlet (1 pu) and recirculation inlet (1 pru) are separate. The recirculation inlet must be dimensioned to allow the introduction of the recirculated enriched phosphoric acid solution P2 at a feed flow rate Q2.

The gas inlet (1 gu), the feed inlet (1 pu) and, if it is separate therefrom, the recirculation inlet (1 pru) are arranged to allow, on the one hand,

-   -   contact between the feed stream F0 and the recirculation stream         F2 to form a stream (F0+F2) of a mixture (P0+P2) of the feed         solution P0 of the contactor and of the recirculated enriched         phosphoric acid solution P2 and, on the other hand,     -   contact of the mixing stream thus formed with the stream of         combustion gases G1.

The gas inlet (1 gu) is preferably arranged so that the combustion gases G1 (which are named G2 during the contact) flow co-currentwise with the phosphoric acid feed stream F0 and recirculation stream F2. However, it is possible to arrange the gas inlet so that the combustion gases flow counter-currentwise relative to the streams F0 and F2.

The gas-acid contactor preferably comprises a packing material, through which percolate the feed stream F0 and the recirculation stream F2 of phosphoric acid solutions. The packing material is preferably arranged on a perforated support, for example a support grate.

The gas-acid contactor (1) comprises one or more enriched phosphoric acid outlets (1 pd, 1 prd), the enriched phosphoric acid outlet(s) (1 pd, 1 prd) are positioned downstream of the gas inlet (1 gu), which is itself positioned downstream of the feed inlet (1 pu) and, if it is separate therefrom, the recirculation inlet (1 pru). The term “downstream” is expressed relative to the direction of flow of the feed stream and the recirculation stream of the phosphoric acid feed solution and the recirculated enriched phosphoric acid feed solution P2 in the gas-acid contactor. The enriched phosphoric acid outlet(s) (1 pd, 1 prd) make it possible to remove the enriched phosphoric acid solution P1 formed in the gas-acid contactor formed by contact between the streams F0 and F2 and the combustion gases G1.

The gas-acid contactor (1) comprises a gas-liquid separator for separating the liquids from the gases after contact between the combustion gases G1 and the solutions P0 and P1. For example, the gas-acid contactor may comprise a demister for recovering any droplets of liquid present in the contacted combustion gas G3 before it leaves via the gas outlet (1 gd).

The gas-acid contactor (1) also comprises a combustion gas outlet (1 gd), for evacuating from the gas-acid contactor the contacted combustion gases G3 after their contact with the mixture of solutions P0 and P2. The device may be followed by a tower for scrubbing the contacted combustion gases G3 located downstream of the combustion gas outlet (1 gd) of the gas-acid contactor, for removing any fluorinated and sulfur-based compounds that the gases contain before releasing them into the atmosphere.

The device is equipped with a combustion gas fluidic connection (6) connecting an end (6 u) coupled to the combustion gas evacuation outlet of the combustion chamber (2), to an end (6 d) coupled to the combustion gas inlet (1 gu) in the gas-acid contactor (1). The temperature in this fluidic connection (6) should preferably be maintained as high as possible so that, at the inlet (1 gu) in the gas-acid contactor, the combustion gases G1 have a temperature that is as close as possible to the temperature Tg1 that they have at the combustion chamber outlet, i.e. about 200 to 600° C.

The device is equipped with a fluidic connection (3, 3 p) connecting an upstream end (3 u) coupled to the enriched phosphoric acid outlet (1 pd) of the gas-acid contactor (1), to a downstream end (3 d) coupled to the spraying inlet (2 pu) of the combustion chamber (2). As the enriched phosphoric acid solution P1 has recovered the majority of the phosphorus molecules carried by the combustion gases G1, the fluidic connection (3, 3 p) makes it possible to reinject these molecules into the combustion chamber so as to obtain a combustion solution P3 which is as rich as possible in P₂O₅. The enriched phosphoric acid solution P1 has a temperature above that of the feed solution P0 of the contactor, which allows better management of the heat energy of the process by injecting into the combustion chamber a solution that is already at a relatively high temperature. In the case where PPA production is desired, the higher concentration of P₂O₅ and the higher temperature of the enriched phosphoric acid solution P1 make it possible to increase the concentration yield in the combustion chamber.

This improvement in the transfer of phosphoric acid molecules and in the concentration yield is made possible by means of the recirculation loop for reintroducing into the gas-acid contactor part of the phosphoric acid stream P1 taken from the same gas-acid contactor. Thus, the device also comprises a recirculation fluidic connection (3 r) connecting an upstream end coupled either

-   -   to a recirculated enriched phosphoric acid outlet (1 prd) of the         gas-acid contactor (1), or     -   to a branching point (5 v) with the first fluidic connection         (3),     -   to a branching point (4 r) with the first fluidic connection (3         u),     -   to a downstream end (3 r) connected to the recirculation inlet         (1 pru) or (1 pu) of the gas-acid contactor.

The device is equipped with means for controlling and maintaining a ratio, Qp/(Qp+Q2), between a spraying mass flow rate Qp flowing in the first fluidic connection (3) and a total mass flow rate (Qp+Q2) defined as the sum of the spraying mass flow rate Qp and of a recirculation mass flow rate Q2 flowing in the recirculation fluidic connection (3 r) at a value of less than 50%, preferably less than 10%, preferably less than 5%, more preferably less than 2.5% and the ratio Qp/(Qp+Q2) has a value of greater than 0.1%, preferably greater than 0.5%.

As illustrated in FIG. 2, the fluidic connections (3 p) and (3 r) may be deconsolidated over their entire length between the gas-acid contactor and the combustion chamber with, on the one hand, the spraying fluidic connection (3 p) connecting a first enriched phosphoric acid outlet (1 pd) to the enriched phosphoric acid inlet (2 pu) in the combustion chamber and, on the other hand, the recirculation fluidic connection (3 r) connecting a second enriched phosphoric acid outlet (1 prd) to the recirculated enriched phosphoric acid inlet (1 pu) of the gas-acid contactor or to the feed connection (3 a) feeding the gas-acid contactor with feed solution P0 from the contactor. Each of the spraying (3 p) and recirculation (3 r) fluidic connections are equipped with a pump (4, 4 r) dimensioned to maintain at a desired value the ratio Qp/(Qp+Q2) or a fluid transfer system.

In an alternative variant illustrated in FIGS. 3 to 5, the gas-acid contactor is equipped with a single outlet (1 pd) for enriched phosphoric acid P1 of the gas-acid contactor which is coupled to a first fluidic connection (3). The upstream parts of the spraying (3 p) and recirculation (3 r) fluidic connections are coupled to a branching point (5), thus forming with the first fluidic connection (3) a T-shaped or Y-shaped branching. In this variant, various means may be used to control and maintain the ratio Qp/(Qp+Q2) at the desired value.

In a first variant illustrated in FIG. 5, the means for ensuring a ratio Qp/(Qp+Q2) at the desired value comprise a pump (4) arranged on the spraying fluidic connection (3 p) and having a capacity for pumping a liquid at a spraying flow rate Qp and a recirculation pump (4 r) arranged on the recirculation fluidic connection (3 r) and having a capacity for pumping a liquid at a recirculation flow rate Q2.

In a second variant illustrated in FIGS. 3 and 4, the means for ensuring a ratio Qp/(Qp+Q2) comprise a pump (4) arranged on the first fluidic connection (3) upstream of the branching point (5) and having a capacity for pumping a liquid at a main flow rate (Qp+Q2) and one or more valves (5 v) (e.g. a three-way valve) arranged at the branching point (5) and making it possible to divide the main flow rate into a spraying flow rate Qp toward the spraying fluidic connection (3 p) and into a recirculation flow rate Q2 toward the recirculation fluidic connection (3 r).

In a third variant (not shown), the means for ensuring the ratio Qp/(Qp+Q2) comprise a pump (4) arranged on the first fluidic connection (3) upstream of the branching point (5) and having a capacity for pumping a liquid at a main flow rate (Qp+Q2) and pipes forming the spraying (3 p) and recirculation (3 r) fluidic connections dimensioned so as to obtain the desired ratio Qp/(Qp+Q2). This solution is less flexible than the first two in the sense that once the pipes have been dimensioned, the ratio Qp/(Qp+Q2) cannot easily be varied, which is not necessarily a problem if the ratio does not need to vary during the lifetime of the device.

In a fourth variant (not shown), the means for ensuring the ratio Qp/(Qp+Q2) comprise a pump (4) arranged on the first fluidic connection (3) upstream of the branching point (5) and having a capacity for pumping a liquid at a main flow rate (Qp+Q2) and forming the spraying (3 p) and recirculation (3 r) fluidic connections and also valves adjusted so as to obtain the desired ratio Qp/(Qp+Q2).

Table 2 lists a series of ranges of values of the various parameters adapted for performing the process of the present invention.

TABLE 2 Examples of values of parameters adapted to the process of the present invention T

xp

Q

° C. %[P

O

] [kg/(h MW)] min max Min max Min max F0 20 200 ≥0% 54% 100 3000 F1 100 300 >F0 60% 800 12

000 F2 100 300 >F0

 F1 60% 300 120000 F

100 300 >F0

 F1 60% 300 3000 F

20 200 ≥1% 50% 5 1500 F

20 300 ≥1% 60% 305 3000 F3 150 700 >F

76% 240 1500 G1 200 600 0.1% 15% — — G3 100 250 <G

 1% — —

indicates data missing or illegible when filed

# Characteristic 1 Gas-acid contactor 1gd Combustion gas outlet of the gas-acid contactor 1gu Combustion gas inlet into the gas-acid contactor 1pd Outlet of enriched phosphoric acid solution, P1, of the gas-acid contactor 1pu Feed inlet for the contact solution, P0, or for the mixture (P0 + P2) into the gas-acid contactor 1pru Inlet for recirculation enriched phosphoric acid, P2, into the gas-acid contactor (optional) 2 Combustion chamber 2c Combustion unit 2pd Combustion outlet, P3, of the combustion chamber 2pdu Entry for residues, Pr, into the combustion chamber 2pu Entry for spraying solution, Pp, into the combustion chamber or combined entry for the direct feed stream and the spraying streams 3 First fluidic connection 3a Feed fluidic connection 3d Downstream end of the first fluidic connection (3) 3p Spraying fluidic connection 3r Recirculation fluidic connection to the gas- acid contactor (1) 3rd Downstream end of the recirculation fluidic connection (3r) 3u Upstream end of the spraying fluidic connection (3p) or of the first fluidic connection (3) 4 Pump 4r Recirculation pump 5v Valve or set of valves (e.g. three-way valve) 6 Combustion gas fluidic connection 6d Combustion gas connection outlet 6u Combustion gas connection inlet 10 Source of combustible for the combustion unit (10) 11 Heat exchanger Fp Enriched phosphoric acid solution spraying stream F2 Recirculated enriched phosphoric acid solution recirculation stream F3 Combustion solution stream Fr Residual stream of residual solution Pr Fp Enriched phosphoric acid solution spraying stream Fm Mixing stream for mixing solution Fm (Fr + Fp) G1 Combustion gas G3 Contacted combustion gases P0 Feed solution P0 + P2 Mixture of the feed solution P0 of the contactor and of the recirculated enriched phosphoric acid solution P2 P1 Enriched phosphoric acid solution P2 Recirculated enriched phosphoric acid solution P3 Combustion solution Pr Residual solution Pp Spraying solution Pm Mixing solution (= Pr + Pp) Q0 Feed flow rate of the feed solution P0 of the contactor Q1 Flow rate of enriched phosphoric acid at the contactor outlet (= Q2 + Qp) Q2 Recirculation flow rate of the recirculated enriched phosphoric acid solution Q3 Flow rate of the combustion solution Qg1 Flow rate of the combustion gases to the gas- acid contactor (1) Qg2 Flow rate of the combustion gases in the gas- acid contactor (1) Gg3 Flow rate of the contacted combustion gases outside the gas-acid contactor (1) Qm Flow rate of the mixing solution Pm Qp Spraying flow rate of the spraying solution Pp Qr Flow rate of the residual solution Pr T0 Temperature of the feed solution P0 of the contactor T1 Temperature of the enriched phosphoric acid solution P1 T2 Temperature of the recirculated enriched phosphoric acid solution P2 T3 Temperature of the combustion solution P3 Tg1 Temperature of the combustion gases G1 Tg3 Temperature of the contacted combustion gases G3 

1. A process for purifying an aqueous residual solution comprising phosphorus molecules and undesirable volatilizable materials, comprising the following steps: (a) introducing into a gas-acid contactor (1) a feed stream F0 of a feed solution P0 which is aqueous and comprising phosphorus molecules preferably in the form of species such as orthophosphate at a mass concentration, xp0, of between 0 and 54% equivalent of P₂O₅ units, (b)introducing into the gas-acid contactor (1) a recirculation stream F2 of recirculated enriched phosphoric acid P2, (c) introducing into the gas-acid contactor (1) combustion gases G1, (d)contacting the feed stream F0 and the recirculation stream F2 and the combustion gases G1 to form in the gas-acid contactor, on the one hand, an enriched phosphoric acid solution P1 comprising a mass concentration, xp1, with a P₂O₅ content which is greater than xp0 (xp1>xp0) and, on the other hand, contacted combustion gases G3, (e) separating the contacted combustion gases G3 from the enriched phosphoric acid solution P1, and then evacuating the contacted combustion gases G3 from the gas-acid contactor (1), and removing the enriched phosphoric acid solution P1 from the gas-acid contactor (1), (f) forming from said enriched phosphoric acid solution P1, on the one hand, a recirculation stream F2 of recirculated enriched phosphoric acid P2 to introduce it into the gas-acid contactor (1) as defined in step (b) and, on the other hand, a spraying stream Fp of the enriched phosphoric acid solution P1 to introduce it into a combustion chamber (2), (a) spraying through a flame burning in the upper part of the combustion chamber (2) a mixing stream Fm of a mixing solution (Pm) comprising phosphorus at a mass concentration, xpm, and undesirable volatilizable materials, the mixing stream being formed by, on the one hand, the enriched phosphoric acid solution P1 and optionally, on the other hand, a residual steam Fr of an aqueous residual solution Pr comprising a mass concentration, xpr, of at least 1% P₂O₅, to: evaporate the water and thus concentrate the mixing solution Pm, optionally oxidize and in any case evaporate the undesirable volatilizable impurities, form combustion gases G1, and forming a combustion solution P3 having a mass concentration, xp3, of P₂O₅ which is higher than the concentration of the mixing solution Pm, and a content of volatilizable impurities which is lower than that of the mixing solution Pm, (g) separating the combustion solution P3 from the combustion gases G1 and recovering the combustion solution P3, and transferring the combustion gases G1 into the gas-acid contactor (1) as defined in step (c).
 2. The process as claimed in claim 1, in which the feed solution F0 comprises a concentration xp0 of between 0.1% and 50% P₂O₅ and in which a flow rate QO of the feed solution F0 into the contactor expressed in nominal power units [MW−1] of the combustion chamber is between 100 and 3000 kg/(h MW).
 3. The process as claimed in claim 1, in which the enriched phosphoric acid solution P1 is identical to the recirculated enriched phosphoric acid solution P2 and comprises a phosphorus concentration xp1 of greater than or equal to 1% P₂O₅ and in which a total flow rate, Q1=(Qp+Q2), of the solution P1 out of the contactor expressed in nominal power units [MW−1] of the combustion chamber is between 600 and 123 000 kg/(h MW) and a ratio, Qp/(Qp+Q2), between the mass flow rate Qp of the spraying stream Fp and the total mass flow rate (Qp+Q2) is preferably less than 50% and in which the ratio Qp/(Qp+Q2) is greater than 0.1%.
 4. The process as claimed in claim 1, in which the residual solution Pr comprises a phosphorus concentration xpr of greater than or equal to 2% P₂O₅, the residual solution Pr comprises a concentration xpv of undesirable volatilizable materials of at least 5 ppm by weight relative to the total weight of the solution, and in which a flow rate Qr of the residual solution Pr in the combustion chamber expressed in nominal power units [MW−1] of the combustion chamber is non-zero and between 5 and 1500 kg/(h MW).
 5. The process as claimed in claim 1, in which a ratio Qe/(Qr+Q0) is between 0 and 99%, and in which the mixing stream (Fm) comprises a phosphorus concentration (xpm) of greater than 1% P₂O₅ (xpm>1% P2O5) and in which Q0, Qp and Qr are mass flow rates of the feed solution (P0), of the enriched phosphoric acid solution P1 and of the residual solution Pr, respectively.
 6. The process as claimed in claim 1, in which the mixing solution Pm comprises a concentration xpm of greater than 2% P₂O₅, the mixing solution Pm comprises a concentration xpv of undesirable volatilizable materials of at least 5 ppm by weight relative to the total weight of the solution, and in which a flow rate Qm of the mixing solution Pm in the combustion chamber expressed as nominal power units [MW−1] of the combustion chamber is between 305 and 3000 kg/(h MW).
 7. The process as claimed in claim 1, in which the combustion solution P3 comprises a phosphorus concentration xp3 of greater than 1% equivalent of P₂O₅ units, and in which the flow rate Q3 of the combustion solution P3 outside the combustion chamber expressed in nominal power units [MW−1] of the combustion chamber is between 240 and 1500 kg/(h MW).
 8. The process as claimed in claim 1, in which the feed stream F0 and the recirculation stream F2 are either mixed before they are introduced into the gas-acid contactor to form a stream of a mixture of the feed solution P0 and of the recirculated enriched phosphoric acid solution P2, or contacted after having been introduced separately into the gas-acid contactor to form a stream of a mixture of the feed solution F0 and of the recirculated enriched phosphoric acid solution P2.
 9. The process as claimed in claim 1, in which a residual flow rate Qr of the residual solution Pr is non-zero, the residual solution Pr and, preferably, the spraying solution Pp comprise undesirable volatilizable materials, and in which the residual stream Fr and the spraying stream Fp are either mixed to form the mixing stream Fm before being sprayed into the flame in the combustion chamber, or sprayed separately into the combustion chamber to form the mixing stream Fm in the flame or just before reaching the flame.
 10. The process as claimed in claim 1, in which the contact between the feed stream F0 and the recirculation stream F2 and the combustion gases G1 in step (d) may take place co-currentwise or counter-currentwise, preferably co-currentwise by flowing from an upper part to a lower part of the gas-acid contactor and in which, during the contact step (d), a ratio (Qg1/(Q0+Q2)) between a mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor (1) and a total mass flow rate (Q0+Q2) of the contact feed stream F0 and the recirculation feed stream F2 introduced into the gas-acid contactor (1), is between 0.1% and 50%.
 11. A device for producing purified phosphoric acid P3 according to the process as claimed in claim 1, comprising: (A) a combustion chamber (2) having: a spraying inlet (2 pu) in the combustion chamber for introducing at a flow rate an enriched phosphoric acid solution P1 in sprayed form into a combustion unit (2 c), a residue inlet (2 pdu) in the combustion chamber or upstream of the spraying inlet (2 pu) for introducing a residual solution Pr or a mixture of residual solutions Pr and of enriched phosphoric acid solution (P1) in sprayed form into a combustion unit (2 c), the combustion unit (2 c) being arranged in the upper part of the combustion chamber, and being capable of forming a flame having a temperature of at least 1500° C. by combustion of a combustible, said combustion unit comprising: a burner, fluid connections between the burner and, on the one hand, an oxygen source and, on the other hand, a combustible source (10) for feeding the flame, a combustion outlet (2 pd) of the combustion chamber for recovering a combustion solution P3 in liquid phase, and arranged downstream of the combustion unit, which is itself arranged downstream of the spraying (2 pu) and residue inlet (2 pdu), an evacuation outlet for combustion gases G1 obtained from the flame, (B) a gas-acid contactor (1) having a feed inlet (1 pu) connected to a source of a feed solution F0, for introducing at a contact feed flow rate Q0 a feed solution F0, a combustion gas inlet (1 gu) for introducing into the gas-acid contactor combustion gases G1 at a flow rate Qg1, a recirculation inlet (1 pru) which is identical to or different from the contact feed inlet (1 pu), for introducing a recirculated enriched phosphoric acid solution P2 at a recirculation flow rate Q2, the feed (1 pu) and/or recirculation (1 pru) inlets and the gas inlet (1 gu) being arranged to allow, on the one hand, contact between the feed stream F0 and the recirculation stream F2 to form a stream of a mixture of the feed solution F0 and of the recirculated enriched phosphoric acid solution P2 and, on the other hand contact of the mixture thus formed with the combustion gases G1, one or more enriched phosphoric acid outlets (1 pd), (C) a combustion gas fluidic connection (6) connecting an end (6 u) coupled to the combustion gas evacuation outlet of the combustion chamber (2), to an end (6 d) coupled to the combustion gas inlet (1 gu) in the gas-acid contactor (1), (D) a first spraying fluidic connection (3 p) connecting an upstream end (3 u) coupled to the enriched phosphoric acid outlet (1 pd) of the gas-acid contactor (1) or to a branching point (5) with a first fluidic connection (3) which is coupled to the enriched phosphoric acid outlet (1 pd), to a downstream end (3 d) coupled to the enriched phosphoric acid inlet (2 pu) in the combustion chamber (2), characterized in that the device also comprises (E) a recirculation fluidic connection (3 r) connecting an upstream end coupled to a recirculated enriched phosphoric acid outlet (1 pd) of the gas-acid contactor (1) or to a branching point (5) with the first fluidic connection (3), to a downstream end coupled to the recirculation inlet (1 pru) of the gas-acid contactor (1) or to a feed connection (3 a) feeding the gas-acid contactor with feed solution F0, and (F) means for controlling and maintaining a ratio, Qp/(Qp+Q2), between a spraying mass flow rate Qp flowing in the spraying fluidic connection (3 p) and a total mass flow rate (Qp+Q2) defined as the sum of the spraying mass flow rate Qp and of a recirculation mass flow rate Q2 flowing in the recirculation fluidic connection (3 r) at a value of less than 50%, and in which the ratio Qp/(Qp+Q2) has a value of greater than 0.1%.
 12. The device as claimed in claim 12, in which the residue inlet (2 pdu) is in fluidic communication with a source of a residual solution Pr which is aqueous and comprises phosphorus molecules in orthophosphate and/or polyphosphate form and undesirable volatilizable materials. 